Where can I get data for a map of all of the letters a historical figure sent?

I make maps and interactives for Slate.com. I thought it might be interesting to make a map showing all of the locations from which a historical figure sent letters. Like a simpler version of this: http://www.stanford.edu/group/toolingup/rplviz/

Do any people or organizations have structured data that would be suitable for this project? I could use, for example, a dataset of an individual's letters that includes sender location, recipient location, and date.

I think you might need to ask yourself "who might have compiled such a dataset, for whom, and why?" This would require at minimum looking at every single letter, so voluminous correspondents (e.g. the Adams dynasty in America) are right out. People who live all their lives in one or a very few places would produce an entirely uninteresting dataset.

So you're looking for someone who wrote some but not too many letters, who traveled enough to make the geographic sources of those letters in some way interesting, and who is prominent enough (or whose travels, Odysseus-style, are intriguing enough) for someone to have compiled precisely that information.